Vehicle drive axles typically include a pair of axle shafts for driving the vehicles wheels. Under normal conditions, when the vehicle is driven along a straight path, the wheels, and thereby both axle shafts, will be turning at approximately the same speed and the same amount of torque will apply to each wheel. However, once the vehicle starts to turn, the outer wheel covers a greater distance than the inner wheel in the same amount of time. Under these circumstances, the outer wheel will have to rotate faster than the inner wheel in order to avoid slip that causes substantial wear of the tires. For this purpose, the drive axle also includes a differential that distributes input speed and torque to the pair of axle shafts. Differentials allow the inner wheel to turn at a slower speed than the outer wheel as the vehicle turns.
In a commonly known differential, the engine power is transmitted from a vehicle drive-shaft to a pinion gear that engages a crown wheel. The crown wheel is rigidly attached to a gear housing that rotates together with the crown wheel. The gear housing includes four differential pinion gears that split the engine torque provided by the pinion gear between the two axle shafts, allowing the two axle shafts to spin at a different speed.
When the vehicle is driven in a straight path, the crown wheel, gear housing and the differential pinions all rotate together as one unit to distribute the power evenly across the two axle shafts. In this case, there is no relative movement between the differential pinion gears. However, when the vehicle turns, the differential pinion gears rotate on their respective shafts to speed up the rotation of one axle (outer wheel) whilst slowing down rotation of the other axle shaft (inner wheel).
In poor road conditions, i.e., slippery or rough road surfaces, the use of a differential can result in loss of control over the vehicle, since the differential mechanism always applies the same torque to both wheels. The maximum amount of torque that can be transferred by the wheels is limited to the greatest amount that will not make the wheels slip. Consequently, if one of the tires is on ice, all of the torque and speed will be transferred to the wheel on ice, leaving the tire spinning on the ice and stopping the vehicle from travelling forward.
As a solution to the aforementioned problem, differential locking mechanisms are known by the art that, when a wheel slips, allow some torque to be transferred to the non-slipping wheel. Such differential locking mechanisms essentially connect the two axle shafts together, such that the axle shafts rotate at the same speed even in poor road conditions. Differential locking mechanisms usually comprise an electric, pneumatic or hydraulic mechanism to lock the axle shafts together. This mechanism can either be activated manually by the vehicle driver or automatically by a control unit of the vehicle.
There are several types of differential lock mechanisms, such as the clutch type limited slip differential, viscous coupling, Torsen differentials and locking differentials. In case of a locking differential, a lock member, such as a shift collar, is provided that locks the gear housing to the axle shafts. The shift collar can be actuated by a differential lock actuator to engage or disengage with the gear housing, in response to a driver-controlled switch or automatic command. Once the shift collar engages with the gear housing, the differential is locked, thereby connecting the two axle shafts together.
Of course, it is desirable to unlock the differential as soon as normal road conditions are reached again. To this end, the shift collar is actuated in order to disengage from the gear housing, enabling the axle shafts to rotate at different speeds once again. It is a commonly known problem that the shift collar, which locks the gear housing to the axle shaft can get stuck to the gear housing and will not release the gear housing even if an automatic or driver controlled switch signal is present. For this reason, lock detection assemblies have been developed that sense the occurrence of a lock condition in a differential locking mechanism. These lock detection assemblies often facilitate a cam mechanism that indicates the position of a push rod of the actuator that is used to move the shift collar between its engaged/disengaged positions. The implementation of such cam assemblies is known to be time consuming and expensive, as very small tolerances need to be met. Furthermore, cam mechanisms are subject to wear and can, therefore, reduce the service life of the detector assembly.